At the end of Going Postal, Moist von Lipwig makes a bet that his post office can deliver a message to a town thousands of miles away before the Grand Trunk company can. Immediately before the race begins, von Lipwig pulls a scam against his rival, Reacher Gilt.
von Lipwig brings an ordinary broom painted to look like a witch's broom. Taking the bait, Gilt assumes that the broom can actually fly and gets angry at the postmaster, demanding the rules be changed. By the time the dust has settled, Gilt has agreed that in exchange for von Lipwig leaving the broom behind, his clacks towers won't use the horses stationed at them. Gilt also agrees to personally bet 100,000 dollars with von Lipwig.
I don't understand why these concessions benefit von Lipwig. In fact, his true plan does not depend on having additional money at stake or preventing Gilt from using his horses at all.
Working with Old Trunk employees, von Lipwig intercepts the message the Grand Trunk sends and replaces it with a message that implicates Gilt and his cronies of fraud and embezzlement. This gives Lord Vetinari the excuse he needs to investigate and take over the Grand Trunk company.
This plan works even if Gilt doesn't put more money on the line; has horses he can use to bypass broken towers; or even uses his horses as a pony express, as von Lipwig claimed to be afraid of.
So how does von Lipwig benefit from having pulled this trick on Gilt?